1. Field of the Invention
The present invention relates to a supercharged internal combustion engine and, more particularly, to control of the output of a supercharged internal combustion engine.
2. Description of the Related Art
It is known that when mounting a reciprocating internal combustion engine equipped with a supercharger such as an exhaust gas turbocharger in an aircraft to drive a propulsion propeller, there are operating regions where a so-called "bootstrap" phenomenon occurs.
Here, a "bootstrap" phenomenon means the phenomenon when the degree of opening of the throttle valve of the engine or the speed setting of the governor is held constant during operation but, nevertheless, the engine speed becomes unstable and there are repeated sharp increases and sharp decreases in the engine speed. This bootstrap phenomenon is believed to occur due to the following reasons.
In a supercharged internal combustion engine, an upper limit is set for the engine supercharging pressure to protect the engine and supercharger. Usually, for the control of the engine supercharging pressure, use is made of a mechanical type wastegate valve (hereinafter abbreviated as WGV) arranged in the exhaust pipe at the inlet of the supercharger or a mechanical type intake relief valve or other supercharging pressure control valve provided in the air passage at the outlet of the supercharger. These supercharging pressure control valves are held in the fully closed condition until the supercharging pressure reaches a certain setting. When the supercharging pressure exceeds the setting, they open to suppress the rise of the supercharging pressure. For example, when the WGV opens, part of the exhaust gas bypasses the supercharger, so an increase of the pressure of the exhaust gas flowing into the supercharger (exhaust gas flow rate) is prevented and a rise of the supercharger speed is suppressed. Therefore, the supercharging pressure will not rise to more than the pressure setting. Further, the intake relief valve similarly opens when the supercharging pressure exceeds the setting and releases part of the supercharger outlet air into the atmosphere to suppress the rise of the supercharging pressure.
FIG. 3(A) shows the characteristic of change of the supercharging pressure PD with respect to the engine speed NE in an engine having such supercharging pressure control valve. As shown in FIG. 3(A), while the engine speed NE is low (in FIG. 3(A), region where 0&lt;NE&lt;NE1), since the pressure (flow rate) of the exhaust gas of the engine is low and the supercharger speed does not rise, the supercharging pressure PD is maintained at a substantially constant low value PD1. When the speed of the supercharger rises along with the engine speed and the engine speed reaches NE1 (FIG. 3(A)), the supercharging pressure PD starts to rise. In the region of NE1.ltoreq.NE&lt;NE2 in FIG. 3(A), the supercharging pressure PD sharply rises along with the rise of the engine speed NE. Further, when the supercharging pressure PD reaches the setting PD2 of the supercharging pressure (NE=NE2), the supercharging pressure control valve opens, so the supercharging pressure PD is held at the constant value PD2 regardless of the engine speed (region of NE.gtoreq.NE2). FIG. 3(B) and FIG. 3(C) show the characteristic of the degree of opening of a supercharging pressure control valve (in this case, a WGV) and the characteristic of the engine output with respect to the speed NE, respectively. As shown in FIG. 3(B), the WGV is maintained fully closed in the region where the engine speed NE is lower than NE2. In the region where the speed NE is higher than NE2, the degree of opening of the WGV is increased along with a rise in the speed NE to hold the supercharging pressure PD at the setting PD2.
Therefore, the engine output HP rises relatively gradually in the region where the speed NE is NE&lt;NE1, but the engine output also rises sharply in accordance with a sharp rise of the supercharging pressure PD in the region of NE1.ltoreq.NE&lt;NE2. Further, in the region of NE.gtoreq.NE2, the supercharging pressure PD is maintained at the constant value PD2, so the increase in the engine output once again becomes gradual. Note that the rated output of an engine for an aircraft is usually set to a speed close to that shown by NER in FIG. 3(C).
However, in an aircraft engine, the engine speed NE is determined by the power consumption of the propeller and the engine output. That is, the power consumption of the propeller increases in proportion to the cube of the speed when the pitch is constant. Therefore, in actual operation, when the engine speed rises, the power consumption of the propeller also increases along with the engine output and the engine is operated at a speed where the power consumption of the propeller and the engine output are balanced. Therefore, in the region where the change in engine output with respect to a change in the engine speed is large (for example, the region of NE1.ltoreq.NE&lt;NE2), the engine output will end up changing by a large amount even when the engine speed changes slightly due to disturbances etc., and the balance of the power consumption of the propeller and the engine output will end up being lost.
For example, in a case where the engine speed rises slightly in this operating region, the power consumption of the propeller does not change that much when the speed rises just slightly yet the engine output rises by a large margin. Therefore, the rise of the engine speed does not stop at the slight amount of rise of the speed which triggered it and rises all at once to the speed where the increase in the power consumption of propeller due to the rise in the speed and the output determined by the engine output characteristic (FIG. 3 (C)) are balanced.
The condition where the engine speed does not stabilize but repeatedly rises and falls in this way even when the degree of opening of the throttle valve is maintained constant is called a "bootstrap". A bootstrap occurs in the region where the change of the engine output is large with respect to the change of the engine speed such as the region of NE1.ltoreq.NE&lt;NE2 in FIG. 3(C), that is, the operating region where the supercharging pressure control valve is generally fully closed. When a bootstrap occurs, the engine speed can no longer be accurately controlled and problems may arise in the operability of the aircraft. Therefore, in the past, the general practice has been to prohibit regular operation in the bootstrap operating region.
The above explanation was made for the case of a fixed pitch propeller, but generally a speed governor using a variable pitch propeller is used for an aircraft engine. A speed governor is a mechanism for adjusting the power consumption of the propeller by changing the pitch of the variable pitch propeller so as to hold a set engine speed. That is, when the propeller pitch changes, the power consumption of the propeller will change even at the same propeller speed. Therefore, when a difference arises between the engine speed during flight and the preset speed, the governor performs control to change the propeller pitch in accordance with the difference between the actual speed and the set speed and hold the engine speed at the set value. In this way, in an engine provided with a speed governor, it is possible to obtain an engine output more stable than the case of a fixed pitch propeller engine.
However, even in an engine provided with such a speed governor, there are cases where bootstrap occurs in a bootstrap operating region where the rate of change of the output with respect to the engine speed is large as mentioned above. That is, in such an engine, when the engine speed rises, the propeller pitch is increased to make the engine speed fall, but in the actual operation, there is a time delay caused by the operational delay etc. of the pitch control mechanism in the time interval from when the rise of the speed is detected to when the propeller pitch increases. Therefore, in the bootstrap operating region where the change of the output with respect to the engine speed is large, the engine speed ends up sharply rising during this delay time and cases arise where bootstrap occurs in the same way as the case of a fixed pitch propeller.
As explained above, bootstrap occurs due to the fact that the characteristic of increase of the engine output due to the rise of the supercharger speed and the characteristic of increase of the engine load (power consumption of the propeller) do not match each other.
While not relating to an aircraft engine, attempts have been made to control the supercharging pressure of the supercharger to make the characteristic of increase of the engine output and the characteristic of increase of the engine load match.
For example, this type of control apparatus is described in Japanese Unexamined Utility Model Publication (Kokai) No. 61-113936. The apparatus of this publication relates to a supercharging pressure control apparatus for a supercharged internal combustion engine used in vehicles. In this vehicular use supercharged internal combustion engine, at the time of vehicle starts and rapid acceleration, the supercharging pressure of the turbocharger rises sharply along with a rise in the engine speed, so the engine output also sharply increases. Therefore, a mismatch occurs between the characteristic of increase of the engine output and the characteristic of increase of the engine load (drive power to drive wheels). This causes slipping of the drive wheels at starts and during acceleration. The apparatus of the above publication is directed to preventing slipping of the drive wheels. The apparatus opens the wastegate valve to correct the supercharging pressure to become lower than the normal pressure setting when the rate of rise of the engine speed exceeds a predetermined value. The apparatus of this publication suppresses sharp rises in the speed of the engine by this correction so as to prevent slipping of the drive wheels from occurring.
The apparatus of the above Japanese Unexamined Utility Model Publication (Kokai) No. 61-113936 is effective in prevention of slipping of drive wheels during vehicle starts and acceleration, but if the apparatus of the publication were applied to a supercharged internal combustion engine for an aircraft, it would not be possible to prevent occurrence of bootstrap.
That is, in the apparatus of the above publication, the wastegate valve is designed to be opened after a sharp rise in the engine speed is detected. However, in an aircraft engine, when a sharp rise of the engine speed occurs, bootstrap is already occurring. Therefore, in the apparatus of this publication, while it is possible to reduce the extent of rise of the supercharging pressure (engine output) after the occurrence of bootstrap, it is not possible to prevent the occurrence of bootstrap itself.